Direct capture of carbon dioxide

ABSTRACT

The present disclosure provides systems and methods for direct air capture of carbon dioxide or other gases through a calcium sorbent in a manner that allows for wide scale, relatively low cost implementation. In particular, a calcium sorbent may be provided as a substantially thin coating on one or more substrates and utilized for direct air capture of carbon dioxide through chemisorption. The carbonated sorbent may be disposed of for sequestration of the carbon dioxide or regenerated with capture of carbon dioxide released from the carbonated sorbent during the regeneration process.

FIELD OF THE DISCLOSURE

The present disclosure relates to systems and methods useful forcapturing carbon dioxide. More particularly, the systems and methods candirectly capture the carbon dioxide from air or another stream throughuse of a sorbent.

BACKGROUND

There is an ongoing effort worldwide to address increasingconcentrations of greenhouse gases, particularly carbon dioxide (CO₂) inthe atmosphere. Although much work has been done to decrease the amountsof such gases that are being released annually into the atmosphere,there is a growing understanding that decreased emissions alone may beinsufficient to address concerns related to climate change. As such,current research is underway for methods for not only reducinggreenhouse gas emissions but also for removing such gases that arealready present in the atmosphere.

Wide scale deployment of current methods for capturing CO₂ from ambientair has been unsuccessful due at least in part to the excessive costassociated with known methods considering that global CO₂ emissions areon the scale of about 37 billion tons per year. Current CO₂ removalmethods are saddled with high capital costs as well as high operatingcosts for various reasons. For example, since overall CO₂ concentrationsin ambient air are very dilute relative to the main constituents (e.g.,nitrogen, oxygen, and argon), previously identified removal methods haverequired high specificity and high efficiency in order to be costeffective. Even though high efficiency removal methods have beenpreviously identified, such methods have lacked the capability to pass asufficient volume of ambient air through the capture systems to make theknown methods simple enough and sufficiently cost-effective forindustrial scale implementation. As a prerequisite for achieving highefficiency, known removal methods have required the use of solventsand/or chemicals that are regenerable in order to offset the high costof the necessary materials. Accordingly, such regenerable solventsand/or chemicals that have been used in known capture methods must beseparated from the captured CO₂ for regeneration, and this separationrequirement also adds additional cost and energy to the processes.

Although there is a strong desire worldwide to implement CO₂ removaltechnology as a means to combat climate change, unreasonably high costshave to date stood in the way of such implementation. Without anaffordable technology for removing existing CO₂ from the atmosphere, theworld will continue struggle to reduce and reverse anthropogenic globalwarming. Accordingly, there remains a need in the field for additionaltechnologies effective for removal of greenhouse gases, including CO₂,from ambient air.

SUMMARY OF THE DISCLOSURE

In one or more embodiments, the present disclosure can provide systemsand methods that are adapted to or configured to capture carbon dioxidedirectly from ambient air. Beneficially, the systems and methods canmaximize the available surface area of a calcium sorbent to increase theamount of carbon dioxide that may be removed from ambient air. Thepresent disclosure also can be effective for minimizing associated costsof an integrated production and logistics system with a passive aircapture process while simultaneously mitigating the accumulation ofcarbon dioxide in the atmosphere.

The presently disclosed systems and methods can provide for highlyefficient, direct removal of carbon dioxide from ambient air at arelatively low cost, particularly when compared to known carbon dioxideremoval systems that require high-cost chemicals that must becontinuously regenerated. More particularly, the present systems andmethods can be adapted to or configured to accelerate carbonation of acalcium sorbent in ambient air, and this can be achieved in someembodiments through providing the sorbent on one or more substrates in amanner that maximizes available sorbent surface area and utilizes adesigned thickness whereby the sorbent may carbonate both rapidly andthoroughly.

The use of calcium hydroxide for chemisorption of carbon dioxide fromair has been previously proposed (see, “Carbon Dioxide Extraction FromAir: Is It An Option?” Lackner, 1999, and “Liquid-Like H₂O AdsorptionLayers to Catalyze the Ca(OH)₂/CO₂ Solid-Gas Reaction and to Form aNon-Protective Solid Product Layer at 20° C.” Beruto and Botter, 2000),but no known system has been shown effective in utilizing theseproperties for a viable direct air capture system with passivecarbonation. Rather, previous systems have relied on active accelerationof direct air capture through carbonators, air contactors, additionalsolvents, pellet reactors, and additional heat and energy input. Suchactive processes, however, add significant cost and risk that isbeneficially avoided by the presently disclosed systems and methods.Even known, passive calcium direct air capture systems (see Erans et al.2019) have been unable to provide a practical, integrated system. Forexample, U.S. Pat. No. 10,570,018 discloses a passive calcium direct aircapture system, but such system still fails to meet necessaryrequirements for practical implementation (i.e., does not allow foraccelerated carbonation at a sufficiently low cost). All of thesefailures, however, may be overcome according to one or more embodimentsof the presently disclosed systems and methods, which provide foraccelerated carbonation at a sufficiently low cost for practicalimplementation.

The presently disclosed systems and methods for direct air capture of atleast carbon dioxide (i.e., other pollutants may similarly be removedfrom ambient air using the present disclosure) overcomes severallimitations in the known processes for removing carbon dioxide from air.For example, known processes require high efficiency, high speedsorbents/chemicals to rapidly remove carbon dioxide from ambient air.Because of the excessive material costs, such processes requireefficient regeneration of the sorbents/chemicals. Likewise, in order toachieve rapid turnover, such processes require active air flow (e.g.,using blowers) to achieve the rapid turnover. The presently disclosedsystems and methods mitigate or completely overcome such limitations. Asis further described herein, the present systems and methods allow forthe use of sorbents/chemicals that are low cost but, due to thepreviously perceived limitations in efficiency, have been disregarded bythe prior art for use in viable air capture systems. The lower costsorbents/chemicals also allow for an integrated sorbent production andcapture process. The present methods and systems likewise can be carriedout efficiently without complete regeneration of the sorbents/chemicals.Further, the present methods and systems can be carried out in theexpress absence (if desired) of any forced air components (e.g.,blowers) since high efficiency may be achieved even when a slowerreaction time it utilized. More particularly, the present systems andmethods provide form implementation of a high surface area configurationthat achieves high efficiency absorption at low cost, even in thepartial or complete absence of sorbent/chemical regeneration and even inthe absence of applied, forced air components.

In one or more embodiments, the present systems and methods mayincorporate the use of a passive calcium chemisorbent carbonationprocess. An example embodiment may utilize one or a plurality ofcirculating and/or stationary hanging substrates that can be randomlydistributed or specifically organized within an enclosed, semi-enclosed,or covered structure. Circulating substrates in particular may utilize avertical, horizontal, and/or passive conveyor system. The calciumsorbent may be provided substantially in a process-ready state—i.e., ina chemical state where the calcium sorbent is ready for undergoing aspontaneous or catalytically driven process to absorb carbon dioxide. Insome embodiments, a calcium sorbent precursor may be treated to providethe process-ready material. For example, calcium carbonate (CaCO₃) maybe subjected to calcining and slaking to form a calcium hydroxide slurryor suspension. Preferably, such processing is carried out with partial,substantially complete, or fully complete capture of any carbon dioxidedriven from the calcium carbonate. Alternatively, or additionally,calcium oxide and/or calcium hydroxide may be provided from a furtherprocess wherein it was produced, preferably with the capture of anyproduced carbon dioxide.

The substrates can be coated with a relatively thin layer of the calciumhydroxide (or other calcium sorbent), which may be particularly in theform of a slurry or suspension. Such coating may be by any suitablemeans but particularly may be carried out as further described herein.After coating, the coated substrates can be positioned in a desired areaof the structure and maintained substantially stationary. Alternatively,the coated substrates may be circulated throughout at least a portion ofthe structure. The coated substrates are subject to contact with ambientair for an amount of time sufficient for evaporation (i.e., release ofH₂O from the calcium hydroxide) and carbonation (i.e., uptake of CO₂from the air) to proceed in a desired amount. The carbonated coating,having absorbed a content of carbon dioxide from the atmosphere, can beremoved from the substrates, and the substantially cleaned substratesmay be re-used. The removed coating may be exported from the site foruse in different process, for an industrial use, for sale, and/or forsequestration. In some embodiments, the calcium sorbent can beregenerated for re-use. For example, the carbonated material may besubjected to further carbonation if desired prior to being processedthrough a calciner to drive off carbon dioxide, which can be capturedfor sequestration or other use (e.g., enhanced oil recovery). Thiscalcination can also generate calcium oxide. Slaking may then be carriedout to form calcium hydroxide to be coated on the substrates for furtherdirect air capture of carbon dioxide.

A system according to the present disclosure may include any combinationof individual components and/or units useful to carry out the processsteps. For example, in some embodiments, a suitable system can comprise:a calciner that is preferably adapted to or configured to capture atleast a portion, substantially all, or completely all of the carbondioxide that is liberated in the calciner. The system likewise caninclude a slaker, a conveyor system (which may be adapted to orconfigured to operate with one or both of vertical and horizontalsegments), and one or a plurality of substrates that are adapted to orconfigured to be suspended at least partially above floor level. Thecomponents of the system may be present with a single structure or aplurality of structures.

The foregoing systems and methods, which are described in greater detailbelow, provide distinct advantages over known uses of calcium sorbentsfor carbon dioxide capture. In particular, the present systems andmethods provide for maximizing the surface area of the calcium sorbent,and thus also maximizing the carbonation efficiency of the calciumsorbent, while minimizing energy costs. In some embodiments, this can beachieved particularly by customizing the coating thickness of thesorbent on the substrate to elicit the most efficient rate of transferof carbon dioxide into the sorbent. This specific design thus increasesthe performance and decreases the cost of the direct air capture system.

The present disclosure provides for even further advantages over knowncarbon dioxide capture systems. For example, passively capturing carbondioxide with utilization of a high surface area sorbent as discussedherein can reduce, substantially eliminate, or completely eliminate theneed for expensive equipment, such as an air contactor, a packed tower,fans, pumps and compressors, a carbonator, and/or a pellet reactor. Thisadvantageous, high surface area arrangement can be achieved, in one ormore embodiments, by utilization of relatively thin layers of thecalcium sorbent on the one or more substrates that are used. An exampleembodiment of a suitable layer thickness can be in the range of about1.5 kg or less of calcium sorbent per square meter of exposed area onthe substrate. This thin layer can be arranged with a relatively highvertical density, such as being greater than five feet tall, whilemaintaining the thinness of each individual layer of the calciumsorbent. This combination of high surface area and low layer thicknesscan provide for direct air capture of carbon dioxide over a reasonablelength of time (e.g., on the scale of several hours to months, dependingupon the exact coating parameters and the desired process throughput)without requiring excessive land coverage. Moreover, thissubstrate-based deposition method allows for thinner application ofcalcium sorbent in a practical and efficient arrangement that solves atleast some of the challenges and issues that have not been addressed byprevious, passive direct air capture systems. Thus, the present systemsand methods can significantly outperform faster, but more capital andenergy intensive, direct air capture systems previously conceived whilealso providing efficiencies not previously attainable in previous,passive direct air capture systems.

Further to the above, the reduction or elimination of any active aircapture mechanism also can reduce associated equipment and costs toproduce the electricity and heat required to run such equipment. As aresult, less carbon dioxide is produced in order to heat and run thecapture system, and the net carbon removal of the present systems cansignificantly exceed that achievable by known systems and methods.

Used calcium sorbent (i.e., sorbent that has already been used topassively capture carbon dioxide) can be calcined in such a way as toseparate the absorbed carbon dioxide for storage and thereby regeneratethe calcium sorbent for further air capture. This regeneration reducesthe need for limestone to produce calcium sorbent. Although calcinationof calcium oxide (CaO) is a common process, its integration with thepresent passive, high density CaO carbonation process is effective toprovide for a significant improvement in cost and energy use of the fullcycle of sorbent use and regeneration compared to other direct aircapture systems. Unlike previously described processes, the systems andmethods of the present disclosure do not require additional chemicals ormaterials in the air capture stage that can complicate the calcinationof the calcium sorbent after it has been utilized for carbon dioxidecapture.

Previously disclosed processes for using calcium oxide or calciumhydroxide for direct air capture were unable to control the space andtime required to scale up the process to an industrial level, even whenpassive systems were proposed. Calcium sorbents have been considered fordirect air capture for approximately 20 years, but never has a systembeen developed that makes it feasible for that calcium to passivelycapture carbon dioxide at a useful scale. The present disclosure solvesthis challenge because of its full-system design for the application ofa thin layer of sorbent to accelerate carbonation and a high densitystorage and logistics process that minimizes space and cost during thecarbonation period, allowing for the passive direct air capture processto operate effectively at industrial scale.

In one or more embodiments, the present disclosure can particularlyprovide methods for direct air capture of carbon dioxide. In exampleembodiments, such methods can comprise: preparing a substantiallycontinuous coating layer of a calcium sorbent material at a density ofless than 10 kilograms per square meter on one or more substrates;subjecting the one or more substrates with the substantially continuouscoating layer of the calcium sorbent material to contact with airincluding carbon dioxide for a time sufficient for the calcium sorbentto react with the carbon dioxide and thereby capture at least a portionof the carbon dioxide from the air and convert at least a portion of thecalcium sorbent to a carbonated form; removing at least a portion of thecalcium sorbent in the carbonated form from the one or more substrates;and processing the calcium sorbent in the carbonated form such that thecarbon dioxide captured from the air is ready for sequestration or otheruse. In further embodiments, the methods can be further defined inrelation to one or more of the following statements, which may becombined in any number and/or order.

The substantially continuous coating layer of the calcium sorbentmaterial can be at a density of about 0.1 ksm to about 5 ksm.

The substantially continuous coating layer of the calcium sorbentmaterial can have an average thickness on the one or more substrates ofless than 2.5 cm.

The substantially continuous coating layer of the calcium sorbentmaterial can have an average thickness on the one or more substrates ofabout 0.01 mm to about 2 cm.

The one or more substrates can be configured substantially as a sheet.

The substantially continuous coating layer of the calcium sorbentmaterial can be configured to exhibit a carbonation rate such that atleast 25% by weight of the calcium sorbent material is carbonated withina time of 96 hours or less.

The substantially continuous coating layer of the calcium sorbentmaterial can be configured to exhibit a carbonation rate such that atleast 50% by weight of the calcium sorbent material is carbonated withina time of about 1 day to about 14 days.

Subjecting the one or more substrates with the substantially continuouscoating layer of the calcium sorbent material to contact with airincluding carbon dioxide can comprise hanging the one or more substrateswith the substantially continuous coating layer of the calcium sorbentmaterial in a location where the substantially continuous coating layerof the calcium sorbent material is in contact with the air.

Removing at least a portion of the calcium sorbent in the carbonatedform from the one or more substrates can comprise subjecting the one ormore substrates to a force sufficient to break the substantiallycontinuous coating layer of the calcium sorbent material and loosen thesubstantially continuous coating layer of the calcium sorbent materialfrom the one or more substrates.

Processing the calcium sorbent in the carbonated form can compriseparticularizing the calcium sorbent in the carbonated form forsequestration of the calcium sorbent in the carbonated form.

Processing the calcium sorbent in the carbonated form can comprisefurther subjecting the calcium sorbent in the carbonated form to ambientair for a time sufficient to increase carbonation percentage.

Processing the calcium sorbent in the carbonated form can comprise:calcining the calcium sorbent in the carbonated form to release carbondioxide therefrom and form calcium oxide; and capturing the carbondioxide released from the calcium sorbent.

The method further can comprise slaking the calcium oxide to form thecalcium sorbent material used in preparing the substantially continuouscoating layer.

The method further can comprise removing a portion of the calciumsorbent in the carbonated form prior to calcining and adding makeuplimestone during calcining.

Preparing the substantially continuous coating layer of the calciumsorbent material can comprise dipping the one or more substrates in areservoir of the calcium sorbent material.

Preparing the substantially continuous coating layer of the calciumsorbent material can comprise dripping or spraying the calcium sorbentmaterial onto the one or more substrates.

In one or more embodiments, the present disclosure further canparticularly provide systems for direct air capture of carbon dioxide.In example embodiments, such systems can comprise: a coating systemconfigured for application of a liquid, calcium sorbent material to oneor more substrates to form a substantially continuous coating layer ofthe calcium sorbent material on the one or more substrates; a storageunit configured for positioning of the one or more substrates for a timewherein the one or more substrates are in contact with air such thatcarbon dioxide in the air reacts with the calcium sorbent material toform carbonated calcium sorbent material; and a collection unitconfigured for removal and collection of the carbonated calcium sorbentmaterial from the one or more substrates. In further embodiments, thesystems can be further defined in relation to one or more of thefollowing statements, which can be combined in any number and/or order.

The coating system can comprise one or more reservoirs of the liquid,calcium sorbent material.

The coating system further can comprise a dipping unit configured fordipping of the one or more substrates into the one or more reservoirs ofthe liquid, calcium sorbent material.

The coating system can comprise a hanging unit configured for retainingthe one or more substrates in a substantially vertical position.

The coating system further can comprise one or more drip pipesconfigured for dripping calcium sorbent material onto the one or moresubstrates.

The system further can comprise a calciner configured to receive thecarbonated calcium sorbent material and convert the carbonated calciumsorbent material into calcium oxide and carbon dioxide.

The system further can comprise a solids separator configured toseparate the calcium oxide from the carbon dioxide.

The system further can comprise a lime slaking unit configured toreceive the calcium oxide and form calcium hydroxide for use as thecalcium sorbent material.

These and other features, aspects, and advantages of the disclosure willbe apparent from a reading of the following detailed descriptiontogether with the accompanying drawings, which are briefly describedbelow. The invention includes any combination of two, three, four, ormore of the above-noted embodiments as well as combinations of any two,three, four, or more features or elements set forth in this disclosure,regardless of whether such features or elements are expressly combinedin a specific embodiment description herein. This disclosure is intendedto be read holistically such that any separable features or elements ofthe disclosed invention, in any of its various aspects and embodiments,should be viewed as intended to be combinable unless the context clearlydictates otherwise.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is an illustration of a substrate coated with a substantiallycontinuous layer of a calcium sorbent material according to an exampleembodiment of the present disclosure.

FIG. 1B is a partial cross-sectional view of a substrate coated with asubstantially continuous layer of a calcium sorbent material accordingto an example embodiment of the present disclosure.

FIG. 2 is an illustration of a substrate being dipped into reservoircontaining a liquid calcium sorbent material so as to form asubstantially continuous layer of the calcium sorbent material on thesubstrate according to an example embodiment of the present disclosure.

FIG. 3 is flowchart showing a system and method for direct air captureof carbon dioxide according to example embodiments of the presentdisclosure.

FIG. 4 is a flowchart showing details of an example of coating andremoval of a sorbent in a system and method for direct air capture ofcarbon dioxide according to example embodiments of the presentdisclosure.

FIG. 5 is a flowchart showing detail of a further example of coating andremoval of a sorbent in a system and method for direct air capture ofcarbon dioxide according to example embodiments of the presentdisclosure.

DETAILED DESCRIPTION

Various aspects of the present disclosure will now be described morefully hereinafter with reference to the accompanying drawings, in whichsome, but not all implementations of the disclosure are shown. Indeed,various implementations of the disclosure may be expressed in manydifferent forms and should not be construed as limited to theimplementations set forth herein; rather, these exemplaryimplementations are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the disclosure to thoseskilled in the art. As used in the specification, and in the appendedclaims, the singular forms “a”, “an”, “the”, include plural referentsunless the context clearly dictates otherwise.

In one or more embodiments, the present disclosure provides systems andmethods for direct capture of carbon dioxide from ambient air oratmospheric air. The systems and methods make use of a calcium sorbentthat preferably is regenerable but that is configured fornon-regenerable uses, if desired. In some embodiments, the calciumsorbent may be configured for undergoing cyclical carbonation andcalcination such that carbon dioxide may be removed from the atmospherethrough absorption by the sorbent and then captured during regenerationof the sorbent. As an example, calcium oxide and/or calcium hydroxidemay be utilized as the calcium sorbent. The calcium sorbent thus can beadapted to or configured to absorb carbon dioxide from ambient airspontaneously or catalytically if desired. Such absorption can beeffective to form a carbonated form of the calcium sorbent, and thecarbonated form can then be regenerated through calcination to drive offthe carbon dioxide, which can be captured in the calcination process.

To form the initial calcium sorbent and/or to regenerate a carbonatedform of the calcium sorbent, a carbonate (e.g., calcium carbonate—CaCO₃)can be injected into a calciner that preferably can be equipped withappropriate components to capture at least a portion of the carbondioxide that is driven off during calcination. The limestone utilized informing the sorbent can be provided in a particulate form to ensure anentrained flow type, rotary kiln, or fluidized bed type calciner can beused for CaO production. In some embodiments, the particulatelimestone/CaCO₃ can have an average size (e.g., the largest measurabledimension of length, width, or thickness for irregularly shapedparticles) of about 1 μm to about 1 mm, about 5 μm to about 750 μm, orabout 10 μm to about 500 μm. As further discussed below, the so-formedcalcium oxide can be further treated prior to being applied to asubstrate for use in the carbonation reaction.

The capture components associated with the calciner are preferablyeffective to capture at least 75%, at least 85%, at least 90%, at least95%, at least 99%, or at least 99.9% of the carbon dioxide that isliberated from the carbonate during calcination. In some embodiments,calcination can be carried out by heating the carbonate to a temperaturein the range of about 700° C. to about 1200° C., about 750° C. to about1100° C., or about 800° C. to about 1000° C. Liberation of carbondioxide can be effective to form calcium oxide (CaO). At least a portionof the carbon dioxide captured from calcination can be utilized forindustrial processes, enhanced oil recovery, or sequestration.Non-limiting examples of capture technologies that may be implemented tocapture the carbon dioxide released in calcination can include oxy-firedcalcination, calcium based chemical looping, amine based solventtechnologies and the like. Calcination for providing low-carbon CaO maybe carried out on-site at the direct air capture facility and thus maybe part of an integrated system for continuous or batch processing. Ifdesired, calcination may be carried out off-site or by a third-party. Assuch, a content of the carbonate may be transported to the calcinationsite for regeneration of the calcium sorbent and carbon capture and thentransported back to the direct air capture facility.

The CaO may remain in its dehydrated form, or it may be hydrated withwater to form calcium hydroxide (Ca(OH)₂), or it may be a mixture ofboth, all of which are referred to as “calcium sorbent” or “limesorbent” interchangeably. As such, a slaking unit may be used inaddition to the calcining unit noted above in preparing the initialsorbent and/or for regenerating the sorbent from a carbonated product.This calcium hydroxide and/or CaO may be further mixed with additionalwater to provide the calcium sorbent in a suitable form for addition tothe one or more substrates. The sorbent prior to addition to thesubstrate(s) thus may be in any one or more of a paste, slurry, spray,or suspension. In addition, in some embodiments, the calcium sorbent maybe prepared in a polymeric form, as a metal organic framework (MOF), oranother suitable mixture and/or molecular structure. The calcium sorbentis preferably provided in any suitable form whereby the calcium sorbentcan capture carbon dioxide from ambient air. For example, calciumhydroxide can capture carbon dioxide from air when the relative humidityis above 40% since it is generally understood that the presence of wateris key for calcium hydroxide to capture carbon dioxide yielding calciumcarbonate. Carbon dioxide in the air can dissolve in water to formcarbonic acid (H₂CO₃), which can dissociate into HCO₃ ⁻ and H⁺, thusallowing for the reaction with calcium to form calcium carbonate andcapture carbon dioxide.

In some embodiments, the calcium sorbent added to the one or moresubstrates may comprise, consist essentially of, or consist ofsubstantially only the calcium material itself and water. In otherembodiments, however, one or more additional chemicals or materials maybe included and may be adapted to or configured to improve the carbondioxide chemisorption properties of the calcium sorbent and/or toimprove the adhesion of the calcium sorbent to the substrate. Likewise,the calcium sorbent may be provided with specific morphologies that canbe adapted to or configured to improve the carbon dioxide chemisorptionproperties of the calcium sorbent. Such improvements may include any oneor more of, for example, reactivity, viscosity, porosity, surface area,morphological stability, and electronegativity, as well as otherbeneficial properties. In an example embodiment, sodium hydroxide inparticular may be added into the sorbent. In other embodiments,potassium hydroxide, magnesium hydroxide, fumed silica, zeolites,magnetic particles, and/or recycled regenerated sorbent may be added. Inaddition to the above mixtures, the calcium sorbent may be formed aspowders, pellets, flakes, slurries, gels, honeycombs, and/or may beprovided in other beneficial geometries. Aging, drying, rehydrating,flexure, flow, vibration, rolling, squeezing, charging, and other suchmanipulations may be applied to the sorbent, as they have beendemonstrated to impact the reactivity and performance of calciumsorbents as applied to the one or more substrates.

Application of the calcium sorbent to the substrate(s) can be carriedout in a coating unit or facility. The calcium sorbent in a compositionand/or form as described above can be applied onto one or moresubstrate(s) in a relatively thin layer. For example, average sorbentlayer thickness across a representative area of the substrate can beless than 2.5 cm, less than 2 cm, less than 1.5 cm, or less than 1 cm(e.g., down to a minimum thickness coating achievable by conventionalcoating methods). In some embodiments, average layer thickness can be inthe range of about 0.01 mm to about 2.25 cm, about 0.01 mm to about 2cm, about 0.01 mm to about 1.5 cm, about 0.01 mm to about 1 cm, about0.01 mm to about 7 mm, about 0.01 mm to about 5 mm, about 0.02 mm toabout 1 mm, or about 0.03 mm to about 0.5 mm. While the foregoing rangesrelate to a variety of useful embodiments according to the presentdisclosure, it is understood that more specific ranges may beimplemented based upon the exact physical nature of the sorbent coatingmaterial. For example, in some embodiments, the calcium sorbent materialmay be provided in a substantially porous form that provides increasedsurface area for reacting with carbon dioxide. In such embodiments,relatively thicker coating layers may be utilized while still providingfor high carbonation percentage, as further discussed below. Forexample, where a relatively thicker coating layer is utilized, theaverage layer thickness can be in the range of about 0.5 mm to about2.25 cm, about 0.75 mm to about 2 cm, about 1 mm to about 1.5 cm, orabout 1.5 mm to about 1 cm. In further example embodiments, relativelythinner coating layers may be utilized and can further simplify theprocess in that additional processing (e.g., to achieve a high porosity,as noted above) can be avoided, and a substantially continuous coatinglayer can be applied in a relatively small average thickness while stillachieving desired carbonation percentages. In such embodiments, where arelatively thinner coating layer is utilized, the average layerthickness can be in the range of about 0.01 mm to about 2 mm, about 0.05mm to about 1.5 mm, or about 0.1 mm to about 1 mm. The average thicknesscan relate to a single layer of the calcium sorbent or may be the totalthickness of a plurality of layers applied to the substrate (e.g., 2, 3,4, or 5 layers). FIGS. 1A and 1B illustrate a representative substrate10 with a coating 20 of the calcium sorbent applied thereto. As seen inFIG. 1A, the coating 20 may cover less than all of a surface 11 of thesubstrate 10, but the coating may cover substantially all of the surfaceif desired. As seen in FIG. 1B, the coating 20 may have a thickness thatis less than a thickness of the substrate; however, substrate thicknessmay vary based upon the specific material used to form the substrate.

Preferably, average sorbent layer thickness is adapted to or configuredto provide a calcium sorbent density within a defined range. As above, auseful calcium sorbent density may vary based upon the average layerthickness that is utilized. Over all desired ranges, calcium sorbentdensity may be in a range such as about 10 kg per square meter (ksm orless), less than 5 ksm, less than 2 ksm, or less than 1 ksm of exposedsubstrate area (e.g., to a minimum of at least 0.02 ksm). In someembodiments, calcium sorbent density on the substrate can be in therange of about 0.05 ksm to about 10 ksm, about 0.1 ksm to about 5 ksm,about 0.2 ksm to about 2 ksm, or about 0.25 ksm to about 1 ksm. Layerthickness may be managed using various mechanisms, such as controllingthe content of water that is mixed with the calcium sorbent to form thecoating mixture. This (or other factors) can be utilized to controlcoating mixture viscosity and thus control the coating thickness of thecoating mixture. In some embodiments, this relatively thin nature of thecalcium sorbent layer can be particularly effective to allow for passivecapture of carbon dioxide. While calcium oxide will capture carbondioxide at ambient conditions, the reaction is rate limited by coatingthickness, and substantially thick layers will essentially ceasereactivity below a certain depth. In some embodiments, however, coatinglayer thickness can be increased by controlling one or more physicalproperties of the coating layer. For example, coating thicknesses in thehigher ends of the aforementioned ranges may be particularly useful whenthe coating layer is provided in a relatively high porosity form.Similarly, the substrate may be provided with a three-dimensionalstructure that allows for greater layering of the sorbent thereon. In anexample embodiment, the calcium sorbent may be prepared substantially inthe form of a foam exhibiting at least a partial, open-cell structureeffective to allow air to penetrate deeper into the layer thickness forreaction of carbon dioxide in the air with the sorbent.

The present systems and methods may be operated within definedcarbonation rates for the sorbent. In some embodiments, the carbonationrate may be maximized so that about 50% or greater, about 60% orgreater, about 70% or greater, or about 80% or greater of the sorbent byweight is carbonated before removal therefrom from the substrate(s). Forexample, removal of the carbonated substrate may take place uponachieving about 60% to about 98%, about 65% to about 95%, or about 75%to about 90% by weight carbonation of the sorbent. Such high levels ofcarbonation would not be expected to be achievable through optimizationof known processes since known processes have either required the use ofother types of sorbents/chemicals or are configured for use withsorbents that are not capable of achieving such high levels ofcarbonation due to structural limitations (e.g., required sorbent layerthickness). In other embodiments, when a high throughput process isdesired, carbonation percentage may be minimized to increase throughputin the system. For example, removal of the carbonated substrate may takeplace upon achieving about 25% to about 75%, about 30% to about 65%, orabout 35% to about 60% by weight carbonation of the sorbent. Byutilizing such concentration limits, turnover of the sorbent may beincreased so that the total mass of carbon dioxide that may be removedby a given system may be maximized. This is because the chemisorptionrate may be significantly faster at lower carbonation percentages forthe sorbent, and the carbonation rate may significantly slow as therelative percentage of the sorbent that has been carbonated increases.

Through application of defined calcium sorbent coating layerthicknesses, coating layer density, and desired carbonation percentage,the calcium sorbent layers applied to one or more substrates may beconfigured to or adapted to provide a carbonation rate within definedparameters. Such carbonation rate would not be expected to be inherentto the calcium sorbent since the carbonation rate will be a function ofthe above-noted factors. In some embodiments, the calcium sorbentcoating formed on the one or more substrates can be configured to oradapted to exhibit a carbonation rate such that at least 25%, at least30%, at least 35%, at least 40%, or at least 45% by weight of thecalcium sorbent coating is carbonated within a time of 96 hours or less,84 hours or less, 72 hours or less, 60 hours or less, 48 hours or less,36 hours or less, or 24 hours or less (e.g., with a minimum carbonationtime of 1 hour). More particularly, the calcium sorbent coating formedon the one or more substrates can be configured to or adapted to exhibita carbonation rate such that about 25% to about 50%, about 25% to about45%, or about 30% to about 45% by weight of the calcium sorbent coatingis carbonated within a time of about 2 hours to about 96 hours, about 4hours to about 84 hours, about 6 hours to about 72 hours, about 8 hoursto about 60 hours, about 10 hours to about 48 hours, or about 12 hoursto about 36 hours. Such carbonation rates can be achieved throughcontrol of one or more of the factors discussed above so that arelatively high throughput system and method may be achieved. This canbe advantageous when the calcium sorbent will be regenerated, and suchhigh throughput can increase the overall volume or mass of carbondioxide removed from the air over a given time period.

In some embodiments, it may be desirable to provide for more completecarbonation of the calcium sorbent coating layer prior to removing thecoating layer from the substrate. Such embodiments can be advantageouswhen the carbonated calcium sorbent will not be regenerated (i.e., willbe sequestered in the form of calcium carbonate that is removed from thesubstrate without calcining to release the carbon dioxide) or wherespace for the system is sufficiently large to allow for longercarbonation times. Accordingly, in such embodiments, the calcium sorbentcoating formed on the one or more substrates can be configured to oradapted to exhibit a carbonation rate such that at least 50%, at least60%, at least 70%, at least 80%, or at least 90% by weight of thecalcium sorbent coating is carbonated within a time of about 0.5 days toabout 14 days, about 1 days to about 12 days, about 1.25 days to about10 days, or about 1.5 days to about 8 days. Evaluating carbonation ratecan be carried out by taking samples of the calcium sorbent coating atvarious times after formation of the coating layer and carrying outchemical analysis of the sorbent (e.g., via mass spectrometry or similaranalytical method effective for identifying chemical composition).

Application of the calcium sorbent to one or more substrates may beachieved in any one or a combination of suitable methods. In someembodiments, the application may be accomplished by dipping a substratedirectly into the calcium sorbent or spraying the calcium sorbent ontothe substrate. In other embodiments, the application may be accomplishedby using a brush, doctor blade, blowing mechanism, dripping, pouring, orany such method that is suitable for the application of a thin layer ofsorbent to the substrate. For example, as illustrated in FIG. 2 , thesubstrate 10 may be dipped in a reservoir 30 (e.g., in the form of atank) containing the calcium sorbent 25. A clamp 40 is attached to thesubstrate 10 for maneuvering the substrate through the dipping process.Alternatively, the substrate 10 may be passed over the reservoir 30 ascalcium sorbent 25 is dripped or sprayed onto the substrate such thatexcess sorbent falls into the reservoir for recirculation to theapplication component (e.g., a sprayer). One or multiple layers may beapplied to a substrate. The application method may be adapted to orconfigured to affect the physical nature of the applied sorbent. Themethod of applying the calcium sorbent to the substrate can vary asdesired to increase throughput; however, certain modes of applying thecalcium sorbent may be preferred in relation improving the reactivity ofthe calcium sorbent with carbon dioxide. For example, blowing of thesorbent onto the substrate may be effective to achieve a coating layerwith increased porosity, which can improve the ability to utilize athicker layer for chemisorption, as described above. In someembodiments, the coating of the calcium sorbent can be specificallycharacterized in relation to being a substantially continuous layer (orlayers) on the substrate. The coating thus may be referenced as being inthe form of a thin film, a sheet, a membrane, or the like. A“substantially” continuous coating or indicates that although certainimperfections in the coating or layer (e.g., cracks, divots, and similardefects) are accounted for, the coating is not present in a form ofdiscrete pieces or particles that exist as individual elements (even ifthe individual elements may be in physical contact with other of theindividual elements). Rather, the coating or layer extends along thelength of the substrate surface as an intact film, sheet, membrane, orthe like. In particular embodiments, the presently disclosed coatingsmay expressly exclude pelletized, lime-based sorbents or otherlime-based sorbents in a particulate form. The use of such, particulateor pelletized sorbents can be disfavored due to the added complexity offorming the particles or pellets, which can require mixing the sorbentwith a filler, binder, or the like, and then spray-drying or otherwiseprocessing the mixture to form the discrete, solid particles or pellets.Such particulate materials likewise can require additional processingthe adhere to a substrate as well as more complex processing to removethe adhered particles for regeneration. Alternatively, such particles orpellets must be positioned in a packed bed reactor so that air withcarbon dioxide can be processed through the packed bed, which againintroduces complexity that is not present according to the methodologyof the present disclosure.

The substrate itself may have no coatings, or it may have adhesives,anti-adhesion, catalysts, polymers, and other additives applied beforeapplication of the calcium sorbent to improve its performance, lifetime,surface area, and/or workability. The substrate surface may be twodimensional (i.e. substantially flat) or three dimensional (i.e.sheared, textured, shaped, curved, etc.) in such a way that improves theapplication, performance, reactivity, surface area, and workability ofthe calcium sorbent. Increases in surface area may be sought toaccelerate the carbonation reaction, and increases in volume of sorbentper unit of substrate material may be sought to decrease the costsassociated with the substrate material. Plastic is one possiblesubstrate material, due to its low costs, structural flexibility,durability and lack of reactivity with calcium. In other embodiments,wood products, foam board, steel, or any such substrate suitable to thepurposes.

The substrate may be adapted or configured to be substantially flat whenlaid on a flat surface or when in a hanging configuration. In otherembodiments, the substrate may be intentionally adapted to or configuredto have a three dimensional shape as noted above. For example, thesubstrate may be shaped into a cylinder, cone, or other shapes, and itis advantageous for all exposed sides of the substrate to be coated inthe calcium sorbent to increase surface area per unit of substratematerial. In some example embodiments, the substrate may be in a rolledconfigured so as to have a substantially spiral cross-section. In otherembodiments, the substrate may be adapted to or configured to have aporous network wherein the pores are sufficiently large to allow notonly the liquid sorbent to flow therethrough and coat the surfaces ofthe substrate but also sufficiently large to allow the carbonatedsorbent to be removed from the substrate. For example, honeycombstructures such as are commonly utilized in catalytic converters (e.g.,for use in the auto industry or for use in exhaust systems in the powerplant industry) may be utilized, and the pore sizes in the honeycombstructure may be suitably sized up to allow for efficient removal of thecarbonated sorbent). Such structures are commonly formed from ceramics,but metal honeycombs may be utilized to improve durability.

The substrate may be rotated, moved, spun, air blown, or otherwisemanipulated to affect the drying of the calcium sorbent on the substratematerial. Likewise, blowers or the like may be utilized to remove excesssorbent from the coated surface and ensure a substantially even andsuitably thin coating of the sorbent on the substrate surface. Asubstrate may be replaced by a rod, a tray, a board, or anothersubstrate that allows for the thin application of calcium sorbent. Insome embodiments, the calcium sorbent may be deposited such that it hassufficient structural integrity on its own to hang, stand, or sitwithout any other substrate or material. Multiple such substrates may becombined in a system to maximize its performance and minimize and wasteor loss of the sorbent material.

After coating of the sorbent onto one or more substrates, the coatedsubstrate(s) may be moved to a storage unit or facility. The calciumsorbent-coated substrate material can be stored or otherwise positionedfor contact with ambient air for a time sufficient to achieve thedesired percentage of carbonation of the sorbent as noted above. Thistime may be referenced as the reaction period or carbonation period. Insome embodiments, the reaction period can range from as little as a fewhours or up to as much as several months, or longer if required. Duringthe reaction period, the sorbent chemisorbs carbon dioxide from thesurrounding air or other concentrated carbon dioxide source. Evaporationwill also occur, both from water added into the calcium sorbent, as wellas water liberated in the reaction from calcium hydroxide with carbondioxide to produce calcium carbonate and water. The layer of sorbentwill exhibit an initial weight decline during evaporation of the waterused in forming the substrate mixture followed by a weight increase fromcarbonation.

The presently disclosed systems and methods beneficially provide forstorage and transportation of the sorbent-coated substrates during thecarbonation period in a manner that achieves particularly desirableresults. One or more substrates with the calcium sorbent applied thereonmay be arranged vertically attached to a conveyor system, hangingstationary from above, supported by other substrates, or held by thesides or bottom of the substrate. This use of vertical space minimizesland area used without significant additional infrastructure and allowsfor two or more sided drying on a conveyor system or the like. In someembodiments, a conveyor system can be adapted to or configured to movethe substrates through the storage unit for at least a portion of or forthe entirety of the duration of the carbonation period. This can bestructured as one continuous conveyor line, a branching conveyor system,or multiple separate conveyor systems. Horizontal stacking of substratescan also be used in addition to or in replacement of the verticalhanging storage, during and after the initial drying process, and eitheron or off of a conveyor system.

In one example embodiment, the calcium sorbent may be first applied on asubstantially vertical hanging substrate to allow for two-sidedapplication (see FIG. 2 ) and, after an initial period of carbonation,the partially carbonated sorbent may be removed and positioned on asubstantially horizontal conveyor system, which can be effective toallow surface area of the sorbent that was attached to the substratematerial to be in contact with ambient air, thus acceleratingcarbonation. In another example embodiment, the calcium sorbent may besolely applied to a vertical substrate and, after the carbonation periodis sufficiently concluded, the at least partially carbonated sorbent maybe removed and proceed to calcination for regeneration of the sorbent.

The storage unit or facility can include suitable coverings that areadapted to or configured to protect the calcium sorbent from weather(e.g., rain) that may disrupt the carbonation process. The storage unitor facility may or may not be closed to the air, depending on theclimate in the region. Temperature, relative humidity, and air flow ofthe storage space may be controlled to optimize carbonation, or they maybe left to fluctuate with ambient conditions. As such, the storage unitor facility may include suitable climate control elements, air intakes,and the like that are adapted to or configured to provide an in-flow ofambient air for carbon dioxide removal. For example, a storage unit orfacility may include one or more air movers (e.g., fans, blowers, or thelike) configured to or adapted to increase air circulation in thestorage unit or facility and ensure that carbon dioxide concentration inthe air proximate to the calcium sorbent do not fall below a range thatmay reduce process efficiency (e.g., average ambient air CO₂ content forair entering the facility minus 5%, minus 10%, minus 15%, or minus 20%).As another example, a storage unit or facility may include one or moreheaters and/or one or more coolers configured to or adapted to regulatetemperature to a desired range for improving process efficiency. Asstill another example, a storage unit or facility may include one ormore humidity regulators, which may be configured to or adapted tomaintain relative humidity (RH) in the storage unit or facility within adesired range (e.g., greater than 40% RH, greater than 50% RH, orgreater than 60% RH, such as in the range of about 45% to about 90%,about 45% to about 80%, or about 50% to about 75% RH) suitable toimprove process efficiency. Additionally, excess carbon dioxide (orother gases) beyond the content present in ambient air may be fed intothe storage unit to accelerate carbonation in some embodiments. In someembodiments, the storage unit or facility may incorporate naturalterrain features such as canyons, water courses, cliffs, or caves tomake the air flow, air temperature, and/or air humidity more suitablefor process performance.

After the calcium sorbent has achieved sufficient carbonation within adesired range, the captured carbon dioxide can be further processed forstorage. In some embodiments, carbonated sorbent may be removed from thesubstrate material for further processing. In other embodiments, asubstrate material may be adapted to or configured to be storedgeologically with the carbonated sorbent, or the substrate may besuitable for being processed through the calciner with the carbonatedsorbent. Thus, the substrate may be reusable or may be sacrificial.Where removal is utilized, removing at least a portion of the calciumsorbent in the carbonated form from the one or more substrates caninclude subjecting the one or more substrates to a force sufficient tobreak the coating layer of the calcium sorbent material and loosen thecoating layer of the calcium sorbent material from the one or moresubstrates. Breaking can indicate breaking into a plurality of piecesfor ease of processing, and breaking the coating layer can improve theability to easily loosen the coating layer from the substrate.

A flexible substrate may be utilized so that bending/flexing of thesubstrate may be sufficient to remove the substantially brittle,carbonated sorbent. As such, subjecting the substrate to a forcesufficient to break and loosen the coating layer can include any forcethat will cause the substrate to bend and/or flex. Alternatively, oradditionally, the force to which the substrate may be subjected caninclude shaking, scraping, blowing, vibrating, rolling, squeeze rolling,shock impulse, electrostatic impulse, electromagnetic impulse, magnetic,and/or a variety of other methods for removing the carbonated sorbentfrom the substrate material. Mechanical force may be desirable in someembodiments since various forms of mechanical force may be applied in acost-efficient manner. In other embodiments, sound or shockforces/impulses may be more easily applied. In some embodiments, removalof sorbent may be enhanced by various compositional and/or surfacetreatments of the substrate. Once removed, the sorbent material may begiven additional time to carbonate given that additional surface areahas been freshly exposed, or it may be substantially immediatelyprocessed.

The carbonated sorbent may be regenerated for reuse by passage through acalciner in substantially the same manner as described above forpreparing the original calcium sorbent material. After carbonation, thesorbent material, which was originally a first chemical composition(e.g., as calcium hydroxide and/or calcium oxide) has been converted toa different chemical composition—namely calcium carbonate. The calciumcarbonate can be processed through the calciner with carbon capture. Byonce again heating up the calcium carbonate to a temperature that isabove about 800° C., carbon dioxide is released and then captured, forsequestration and utilization. The captured carbon dioxide issubstantially or completely carbon dioxide that was withdrawn from theambient air. In addition, calcium oxide is once again formed, which canbe fed back in again as the input for the calcium sorbent, reducing theneed for additional limestone input.

This re-calcining (or regeneration) portion of the process sharessimilarities with calcium looping. Unlike calcium looping, no carbonatoris needed, as carbonation occurs through prolonged exposure to the air.The behavior of calcium as it is looped is well understood, and calciumoxide is known to continue chemisorbing carbon dioxide as it is loopedfrom calcium oxide to calcium carbonate and back to calcium oxide, butalso to lose its reactivity after some number of loops. For example,carbonation conversion can drop to under 50% after as few as fivelooping cycles with a calciner and a carbonator in succession. Thecurrently disclosed systems and methods, however, can mitigate suchsorbent deactivation. In particular, by at least partially slaking thecalcium oxide, water is able to facilitate the reaction between calciumoxide and carbon dioxide. Further, elimination of the carbonationreactor (which is typically operated at about 650° C.) mitigatesattrition and sinter issues. Still further, the residence time of thecalcium hydroxide and carbon dioxide reaction in the current systems andmethods is significantly longer than residence time of calcium oxide andcarbon dioxide in a typical high temperature carbonator. The cyclesbetween carbonation and calcination can be from a few cycles to hundredsof cycles, to reduce the cost of limestone.

To the extent that calcium sorbent deactivation may occur so thatefficiency drops to a defined level, such deactivated sorbent may bedisposed of either in the form of any one or more of CaO, Ca(OH)₂, andCaCO₃. The waste material may be utilized as products, for industriessuch as cement, agriculture, or road aggregate, where chemically similarCaO and limestone are used as inputs. Alternatively, the waste can beprocessed for disposition of the material. To the extent that CaCO₃ isdisposed, it will also serve as mineral sequestration for carbon dioxidefrom the air, keeping it from the atmosphere for millennia in the formof stone. As such, at least a portion of the calcium carbonate removedfrom the substrate(s) after carbonation has been carried out may bedisposed of in a manner that sequesters the captured carbon dioxide. Insome cases, the calcium sorbent may skip the re-calcination stage afterthe very first carbonation, and be immediately stored as in the groundas a stable non-toxic mineral, CaCO₃, which is formed duringcarbonation. Because of the passive low input system developed herein,this mineral carbon dioxide storage is potentially attractive in regionswith low cost limestone and inadequate geology for traditional carbondioxide sequestration.

An example embodiment of an overall system and method according to thepresent disclosure is provided in FIG. 3 . As shown therein, the examplesystems and methods can utilize relatively tall substrates (e.g.,“sheets”) that are dipped into a reservoir (i.e., a “dipping vat”)containing the liquid, calcium sorbent (e.g., in the form of calciumhydroxide) as they are transported by a vertical conveyor. Thosesubstrates are hanging from the ceiling, and the conveyor transportsthem during carbonation until the carbonated sorbent is removed forfurther processing.

In the embodiment of FIG. 3 , a coating system 32 may include a dippingunit 35 for dipping of the substrates 10 (e.g., the hanging sheets) intothe reservoir 30 (e.g., the dipping vat). The dipping unit 35 maycomprise a portion of the vertical conveyors 50 that are angularlyoriented to allow the substrates 10 to move downward into the reservoir30 and then upward out of the reservoir. The coating system 32 maycomprise further components in addition to the reservoir 30, such assprayers or drip components, for applying the calcium sorbent by methodsother than dipping. In other embodiments, the dipping unit 35 maycomprise additional components configured for individually loweringindividual substrates 10 into the reservoir 30. The substrates 10exiting the dipping unit are in the form of coated substrates 52 (e.g.,lime coated sheets) with a layer of the calcium sorbent coated thereon.The coated substrates 52 are then positioned in a storage unit 54. Thestorage unit 54 may consist essentially of the conveyors 50 in asheltered location between the dipping unit 35 and the collection unit60. Alternatively, the storage unit 54 may be a building or room(s) in abuilding where the coated substrates 52 can be subjected to carbonationwherein ambient CO₂ from air reacts with the calcium sorbent and waterevaporates from the calcium sorbent. After sufficient carbonation hasoccurred, the coated substrates 52 are moved to the collection unit 60(e.g., the lime removal unit). Therein, the carbonated calcium sorbentis removed from the substrates 10. The substrates 10 are released forre-use in the dipping unit 35, and the carbonated calcium sorbent (whichcan include CaCO₃ and unreacted Ca(OH)₂) is moved along a horizontalconveyor 65 during which time partially carbonized calcium sorbent canundergo further carbonization such that at least a portion of theunreacted calcium hydroxide is carbonated. All or part of the carbonatedcalcium sorbent can be removed for disposal such that the carbon dioxideremoved from the air is sequestered in the formed calcium carbonate. Allor part of the carbonated calcium sorbent likewise can be sent to acalciner 70 where carbon dioxide can be liberated to again form calciumoxide, which can be removed in a solids separator 75 to provide calciumoxide to a lime slaker 80 to form the calcium hydroxide slurry for inputto the reservoir 30 for use as the calcium sorbent.

FIG. 3 illustrates a fully implementable system and method for carbondioxide capture, but it is understood that only portions of theillustrated components and steps may be implemented to carry outdifferent embodiments of the present disclosure. Thus, FIG. 3 isprovided such that a skilled person utilizing the present disclosure mayimmediately recognize various combinations of the illustrated componentsand steps to achieve the different embodiments. For example, in one ormore embodiments, the method and system illustrated in FIG. 3 may beimplemented under one or more of the following conditions.

-   -   The calciner 70 and the solids separator 75 may be absent from        the system. Instead, calcium oxide may be input directly to the        lime slaker 80. In such embodiments, lime removed from the        substrates in the collection unit 60 may be exported for        sequestration and/or for delivery to a third party for        re-calcination with carbon capture. Likewise, the lime slaker 80        may be absent, and calcium hydroxide may be sourced directly for        input to the reservoir 30.    -   The reservoir 30 may be replaced with any further components        suitable for applying the calcium sorbent to the substrates 10.    -   The “sheets” may be replaced with any other, suitable substrate        material as otherwise described herein.    -   The lime coated sheets 52 may be circulated through the vertical        conveyors for carbonation without any intermediate processing in        the collection unit 60.    -   The CaCO₃ and Ca(OH)₂ withdrawn from the collection unit 60 may        be sent directly to re-calcination without undergoing additional        carbonation.    -   The processing may be substantially continuous in that        application of calcium sorbent, processing of the coated        substrates for carbonation, and removal of the carbonated        sorbent in the collection unit 60 may be carried out without        interruption other than requisite maintenance or scheduled        downtime. For example, the scale of the system may be        sufficiently large such that a single substrate may only cycle        from exiting the coating system 32 to re-entry to the coating        system over a length of time that is sufficient for the desired        level of carbonation to occur. This may be on the order of        several hours to several days. As such, the system may be        continuously operated.    -   The vertical conveyor may include a sitting station at some        point between the coating system 32 and the collection unit 60.        In this manner, coated substrates 52 may be off-loaded in the        sitting station for carbonation to occur while other substrates        are processed through the system. The sitting station may be        intermittently completely or partially emptied of coated        substrates that have already undergone carbonation and        re-populated with freshly coated substrates.    -   The process may be operated in a batch mode wherein substrates        are coated as a batch, stored for carbonation as a batch, and        then processed for lime removal as a batch.    -   The process may be operated through multiple coating and        carbonation steps without removal of the coating layer from the        substrate. For example, a substrate may be coated with a        relatively thin layer of the calcium sorbent material, processed        to allow carbonation to occur, re-coated so that an additional,        relatively thin layer of the calcium sorbent material is added        over the carbonized layer, and so on until a relatively thick        coating of multiple, separately carbonized layers are present on        the substrate. The coated substrate may then be subject to        removal of the coating layers.

Further example embodiments are illustrated in FIG. 4 and FIG. 5 . Withreference to FIG. 4 , the core system as illustrated in FIG. 3 canremain substantially unchanged, but the vertical conveyor system can bereplaced with stationary sheets. In such embodiments, calcium sorbentcan be applied through a pipe and drip system such that the substratesremain stationary while the calcium sorbent is moved through the systemfirst in the form of calcium hydroxide and second in the form ofcarbonated lime. In FIG. 4 , the calcium sorbent can be pumped orotherwise conveyed from a reservoir 30 through one or more lines 100 toone or more drip pipes 110 including one or more perforations 115 fordripping fresh calcium sorbent onto the substrates 200. After thecalcium sorbent has undergone carbonation, the carbonated lime may beremoved from the substrates 200 (e.g., via a shaking system 155integrated with the hanging unit 150) such that the carbonated limefalls onto a horizontal conveyor 165. The conveyor can be configured todeliver carbonated lime to a calciner 70 with carbon capture, which canbe linked to a solids separator 75 and a lime slaker 80 to delivercalcium sorbent to the reservoir 30, similarly to what is illustrated inFIG. 3 . Carbon dioxide and material flows are not shown in FIG. 4 , butit is understood that such material flows can be substantially identicalto what is described in relation to FIG. 3 .

The embodiment of FIG. 3 can be modified to yield further exampleembodiments for implementation of a system and method as describedherein. For example, as illustrated in FIG. 5 , the elements of the dripsystem from FIG. 4 remain, but a gravity based collection system 190 isutilized to replace the horizontal conveyor. After carbonated sorbent isremoved from the substrates 200, it falls onto a slanted surface of thegravity-based collection system 190 such that the solid product willcollect in high enough density to allow for easy transport back into thecalciner 70.

The present systems and methods can achieve net carbon removal from theatmosphere because the net capture of CO₂ from the air by the calciumsorbent is greater than any CO₂ emissions from the process. Net carbonflow in an example embodiment of the present disclosure is shown in thefollowing Table for a system with a 90% carbon capture calciner and an85% calcium sorbent carbonation rate.

TABLE CO₂ captured per ton of 0.67 tons CaO in sorbent CO₂ emitted bycalciner per 0.091 tons MT of CaO produced. All other embodied CO₂emissions, .01 tons per metric ton of CaO produced Net CO₂ removal fromatmosphere .57 tons per metric ton of CaO processed Net direct aircapture CO₂ per 0.65 tons metric ton of CaO Land required per metric tonof .00024 acres net direct air capture CO₂

Experimental data has demonstrated the ability to achieve >70%carbonation over a time period of approximately 3 days, with 0.3 kg ofcalcium sorbent per square meter of exposed area. This pace andconversion rate would allow for approximately 100,000 metric tons (MT)of net direct air capture to occur using less than 25 acres of land.This land use intensity is well below what's required to enable billionsof tons of CO₂ to be captured from the air without impinging on otherland uses or running out of suitable area near limestone and CO₂storage.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andassociated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

Use of the words “about” and “substantially” herein are understood tomean that values that are listed as “about” a certain value or“substantially” a certain value may vary by an industry recognizedtolerance level for the specified value. When an industry recognizedtolerance is unavailable, it is understood that such terminology mayindicate that an acceptable value may be vary ±3%, ±2%, or ±1% from thespecifically listed value. Likewise, in some embodiments, the listedvalue may be exact, if desired, and variations above or below the listedvalue may be expressly excluded.

1. A method for direct air capture of carbon dioxide, the methodcomprising: preparing a substantially continuous coating layer of acalcium sorbent material at a density of less than 10 kilograms persquare meter on one or more substrates; subjecting the one or moresubstrates with the substantially continuous coating layer of thecalcium sorbent material to contact with air including carbon dioxidefor a time sufficient for the calcium sorbent to react with the carbondioxide and thereby capture at least a portion of the carbon dioxidefrom the air and convert at least a portion of the calcium sorbent to acarbonated form; removing at least a portion of the calcium sorbent inthe carbonated form from the one or more substrates; and processing thecalcium sorbent in the carbonated form such that the carbon dioxidecaptured from the air is ready for sequestration or other use.
 2. Themethod of claim 1, wherein one or both of the following conditions aremet: the substantially continuous coating layer of the calcium sorbentmaterial is at a density of about 0.1 ksm to about 5 ksm; thesubstantially continuous coating layer of the calcium sorbent materialhas an average thickness on the one or more substrates of less than 2.5cm.
 3. (canceled)
 4. The method of claim 1, wherein the substantiallycontinuous coating layer of the calcium sorbent material has an averagethickness on the one or more substrates of about 0.01 mm to about 2 cm.5. The method of claim 1, wherein the one or more substrates isconfigured substantially as a sheet.
 6. The method of claim 1, whereinone or both of the following conditions is met: the substantiallycontinuous coating layer of the calcium sorbent material is configuredto exhibit a carbonation rate such that at least 25% by weight of thecalcium sorbent material is carbonated within a time of 96 hours orless; the substantially continuous coating layer of the calcium sorbentmaterial is configured to exhibit a carbonation rate such that at least50% by weight of the calcium sorbent material is carbonated within atime of about 1 day to about 14 days.
 7. (canceled)
 8. The method ofclaim 1, wherein subjecting the one or more substrates with thesubstantially continuous coating layer of the calcium sorbent materialto contact with air including carbon dioxide comprises hanging the oneor more substrates with the substantially continuous coating layer ofthe calcium sorbent material in a location where the substantiallycontinuous coating layer of the calcium sorbent material is in contactwith the air.
 9. The method of claim 1, wherein removing at least aportion of the calcium sorbent in the carbonated form from the one ormore substrates comprises subjecting the one or more substrates to aforce sufficient to break the substantially continuous coating layer ofthe calcium sorbent material and loosen the substantially continuouscoating layer of the calcium sorbent material from the one or moresubstrates.
 10. The method of claim 1, wherein processing the calciumsorbent in the carbonated form comprises particularizing the calciumsorbent in the carbonated form for sequestration of the calcium sorbentin the carbonated form.
 11. The method of claim 1, wherein processingthe calcium sorbent in the carbonated form comprises further subjectingthe calcium sorbent in the carbonated form to ambient air for a timesufficient to increase carbonation percentage.
 12. The method of claim1, wherein processing the calcium sorbent in the carbonated formcomprises: calcining the calcium sorbent in the carbonated form torelease carbon dioxide therefrom and form calcium oxide; and capturingthe carbon dioxide released from the calcium sorbent.
 13. The method ofclaim 12, further comprising slaking the calcium oxide to form thecalcium sorbent material used in preparing the substantially continuouscoating layer.
 14. The method of claim 12, further comprising removing aportion of the calcium sorbent in the carbonated form prior to calciningand adding makeup limestone during calcining.
 15. The method of claim 1,wherein one or both of the following conditions are met: preparing thesubstantially continuous coating layer of the calcium sorbent materialcomprises dipping the one or more substrates in a reservoir of thecalcium sorbent material; preparing the substantially continuous coatinglayer of the calcium sorbent material comprises dripping or spraying thecalcium sorbent material onto the one or more substrates.
 16. (canceled)17. A system for direct air capture of carbon dioxide, the systemcomprising: a coating system configured for application of a liquid,calcium sorbent material to one or more substrates to form asubstantially continuous coating layer of the calcium sorbent materialon the one or more substrates; a storage unit configured for positioningof the one or more substrates for a time wherein the one or moresubstrates are in contact with air such that carbon dioxide in the airreacts with the calcium sorbent material to form carbonated calciumsorbent material; and a collection unit configured for removal andcollection of the carbonated calcium sorbent material from the one ormore substrates.
 18. The system of claim 17, wherein the coating systemcomprises one or more reservoirs of the liquid, calcium sorbentmaterial.
 19. The system of claim 17, wherein the coating system furthercomprises one or both of a dipping unit configured for dipping of theone or more substrates into the one or more reservoirs of the liquid,calcium sorbent material and one or more drip pipes configured fordripping calcium sorbent material onto the one or more substrates. 20.The system of claim 17, wherein the coating system comprises a hangingunit configured for retaining the one or more substrates in asubstantially vertical position.
 21. (canceled)
 22. The system of claim17, further comprising a calciner configured to receive the carbonatedcalcium sorbent material and convert the carbonated calcium sorbentmaterial into calcium oxide and carbon dioxide.
 23. The system of claim22, further comprising a solids separator configured to separate thecalcium oxide from the carbon dioxide.
 24. The system of claim 23,further comprising a lime slaking unit configured to receive the calciumoxide and form calcium hydroxide for use as the calcium sorbentmaterial.